You'll almost definitely want Fastlane but I'm sort of confused by the question about whether or not you should buy it up front. Why stress about that?
Unless there's a special deal online where it ends up being cheaper to purchase in advance Cedar Fair makes it very easy to walk into almost any retail location and buy the wristband in a matter of seconds. If you buy online you still need to bring the voucher so if the price is a wash (and it often is), you might as well buy it in the park if it gives you peace of mind to wait. The transaction will take the same amount of time either way. Cedar Fair makes it really easy. If you want to ride something and the line looks long, go to the nearest store and buy a wristband. Simple